Add ConstantScanner to Zend\Code\Scanner

Fix bug introduced in #4457 and add some new features.

* Can parse constants of type float, number or string
* Can parse constants with docblock on previous line
* Can parse constants with self:: reference
* Added has* methods for constant and property
* Updated docblocks

There is one BC break. Method getConstants() don't return constant names now, but return instance of ConstantScanner.